Honestly, moisture is the classic cause of this sort of thing. Usually when someone complains and you mention moisture, then it's, Oh, yeah, I was carrying it in my pocket and I was all sweaty.... or something like that. Another of the typical scenarios is a very fogged up shower room.

I don't think the damp night air would do it. I believe it takes condensation, not just humid air.

I don't like saying this, but it may be time to give up the life saving measures and get another device.

Just curious, have you had this blackberry since it was new? Or did you get it second hand? I don't know, but I'm wondering if it got wet before you got it and you are seeing the effects of corrosion after the fact.